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Gravity Wells (how do you beat them)

  • Thread starter Thread starter Pickles
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The main problem with Ion Drives is that you are never going to be able to get them to give a thrust even remotely approaching 1G, especially in an atmosphere. Most current Ion Drives are limited by the amount of voltage they can crank out before Vaccum arcing occurs: arcing occurs at much lower voltages in an atmosphere...

So Ion Engines are restricted (by that nasty thing called "Physics" to microgravity vaccum environments.

Scott Martin
 
At least every reaction drive is based on the formula Thrust = Massflow * velocity.
Even if we would be able to create some fancy thrusting ion drive, it either would result into a fancy massflow or fancy velocities of the output stream.

Hard Times presents the scenario, to use ramjet designs (COACC could be used here, too) to reach athmosphere edge in a considerable time and then use a secondary "no-oxygen-breather" propulsion to reach high orbit.

Perhaps we roughly could keep most starship designs in a hard SF environment and live with some new consequences (Heplar environment devastation) but need to create a couple of new designs for the planet-space interface.

I will try to create a few designs (perhaps a ramjet landing shuttle and a hybrid "Cross-Country Trader").
 
Hey Mert

Give me to the end of next week before spending too much time on this: I'm working on "modular" FF&S components, so you may just want to design a platform around a "standard" module size: my stuff is fairly "hard" SF (except that I use HEPLAR...) so this could save you some work. Failing that, assume that you are hauling a cargo container with dimensions of 6M x 7M x 10M (2 decks high, 7M wide, 10M long, 30dTons total) My universe actually uses just cubic meters since Dtons don't fit well with the metric system (you keep getting factors of 7 in your dimensions, and my pocket empire is solomani-derived, from STL ships and thus "uncontaminated" by the bizzarre Vilani practice of using Dtons...)

On the flip side, HEPLAR probably isn't the environmental bugaboo that you are making it out to be. It shouldn't cause too much primary radiation from the exhaust (mostly alpha and other short-lived high energy EM radiation). If the actual landing site(s) is a big graphite pad (since carbon isn't very subject to "neutron poisoning") you should be able to avoid long-term increases in background radiation for your "downport".

Isn't there a thread about uses of Coal in high TL's somewhere ;)

Other "environmental" concerns are likely to be similar to those around any major airport (bird kills etc) but the chemical toxicity issues will be alleviated with "clean" fusion power. You will want to put this a LONG way away from population centers, to keep from being sued by folks who insist on staring at the drive plume (and will suffer "welders blindness" as a result, with basically the same cause) This means that you probably want to use an island as a downport, I can go into reasons if anyone is interested.

Scott Martin
 
Originally posted by Scott Martin:
The main problem with Ion Drives is that you are never going to be able to get them to give a thrust even remotely approaching 1G, especially in an atmosphere. Most current Ion Drives are limited by the amount of voltage they can crank out before Vaccum arcing occurs: arcing occurs at much lower voltages in an atmosphere...

So Ion Engines are restricted (by that nasty thing called "Physics" to microgravity vaccum environments.

Scott Martin
Click to expand...
I must respectfully agree and disagree. I agree the that achieving high G is a problem (of physics) for ion drives, but you don't need high G to get to orbit. You can get to orbit with much lower G it just will kill you if you spend too much time in the radiation belts.

I disagree that ion drives are restricted to vacuum by physics, ion drives will work in "atmosphere" and the arcing is with respect to the gas inside the engine. Diffusion of atmosphere into the engine is minimal against the outgoing "jet." Arching does restrict voltage gradient but the restriction does not mean they will not work in atmosphere.
 
Ptah:

I am suitably chastized (and feeling dumb) for not remembering vapour pressure inside the accellerator chamber.

If you have <1G of thrust you don't get off the ground. even if you are using a lifting body you're going to lose the lifting characteristics of the atmosphere long before you get a significant gravity reduction due to altitude.

The radiation belts are not an issue if you can't get there ;)

Getting high thrust (volume of reactant per unit time * exhaust velocity of thrust) is rather difficult in the Real World(tm) because you either use low volumes of reactant / reaction mass or somehow find a way around the insanely high temperatures generated from high energy / high throughput thrusters.

You could revert to using a solid rocket booster to punch out past the atmosphere (and radiation belts) but you're going to take a large penalty in carrying capacity for that. Alternatives could include triple staging (launching from an aircraft, "conventional" rocket boost then ion thrust once out of the gravity well) to reduce the volume / mass penalty but I don't think you can get single stage to orbit with an ion thruster without it "becoming" HEPLAR (or at least having all of the nasty side effects plus a few)

HEPLAR exhausts protons and high energy EM radiation: Ion drives tend to accellerate heavy nucleii at similar velocities, which (IMO) would become an isotope enhancing particle accellerator putting you back to "environmentally unfriendly" ;)

Scott Martin
 
HEPlaR is an exhaust problem because it has a power output of roughly 20MW per newton of thrust (200GW/ton), with the result that even a small drive can level cities. It might not be a horrible radiation source per se, but that won't help the vaporized people much.
 
Originally posted by Anthony:
HEPlaR is an exhaust problem because it has a power output of roughly 20MW per newton of thrust (200GW/ton), with the result that even a small drive can level cities. It might not be a horrible radiation source per se, but that won't help the vaporized people much.
Click to expand...
But that makes it self-limiting in an atmosphere - in the first few microseconds of operation the energy released is going to destroy the HEPlaR assembly, and possibly the craft using it.

IMTU the 'fusion jet' (heating atmo with the fusion pp) and low-powered HEPlaR is used to exit atmo in combination with CG.

Starviking
 
We're wandering off topic, since this TU has already stated that it won't be using HEPLAR (my bad for distracting folks)

<distraction>
If the HEPLAR drive can operate in a vaccum, then the thermal issues of operating in an atmosphere are not an issue. In point of fact atmospheric use should be easier, since you get to use convective cooling, while vaccum use limits you to black body radiation.

Several folks have tried to determine order-of-magnitude limits on HEPLAR using "Real World" physics. One of the issues has been that the exhaust chamber cannot be made of anything approaching "real" matter because we don't know of anything that is even within a couple of orders of magnitude for thermal transfer and radiation. Even if drive efficiency approaches 100% efficiency (last calcs I saw assumed that only 1% of exhaust energy was lost as heat to the reaction chamber, and assumed that the input energy = output energy) we still vaporize the drive chamber in very short order.

If you're going to shoot down HEPLAR in atmosphere due to thermal issues, you need to realize that it won't work in a vaccum without some significant handwavium, and I don't see any way to keep that handwavium / unobtanuim from allowing it to work in an atmosphere.
</distraction>

This doesn't change Anthony's point, that a running nuclear rocket (HEPLAR is basically "Fusion Rocket +") is a potent weapon at "point blank" ranges (and anything on a planetary scale is "point blank" in this context) and any reasonably efficient reaction drive will have exactly the same problems.

Scott Martin
 
Hi all.

My biggest problem with reactionless drives (like thrusters) is where all the energy goes, energy is consumed and it should end up somewhere too.

I would say that any ship capable of landing on a planet would have some sort of jet engine, or other drive capable of manipulating the atmosphere. Once out of the atmosphere it would need some sort of drive propelling the ship forward by accelerating stuff backwards.

So to examples of that would be:

Gurps Space have superscience reaction drives they call Fusion rockets, they use all kinds of handwaveium to work but they are reaction drives. The fusion rockets deliver( at GurpsTL10 (TL12?)) 160 tons of thrust per ton of engine, while consuming 27 tons/hour of propellant (water). Thus a ship weighing 1600tons could accelerate at 1G for (1590/27) 59 hours, assuming it has nothing but engine and fuel/propellant. Should work fine if you use in system jumps and only use the manoeuver drive to get to a safe distance from the gravity well.

Gurps Space also have, what they authors believe is the best possible "realistic" interplanetary-drive, called Anti-Matter pion drives. It delivers 0,02 tons of thrust/ton and consumes 0,00000017 tons of AM/hour. So a ship capable of 0,01G would first of all be half engine. It could sustain acc 0,01G for (AM tanks can store 0,00001 tons AM/ton tank) 1225 days if the other half of the ship is AM storage. But it won’t do much if you want to beat the planetary gravity well.
 
Given a number of different engine options for reaching orbit, why is one more destructive than another? If each is producing the same amount of thrust, the kinetic energy blowing out of the rear should be the same, right? Is the difference in destructiveness a question of heat? Radiation?

This is not a rhetorical question, I am a genuine rocket propulsion ignoramus. :confused:

If the danger zone is limited to a fairly small area behing the bell, perhaps horizontal take-off would be better than vertical. Then the plume could dissipate rather than burn a hole in the launch pad.
 
Brom: a multi-stage to orbit nuclear atmospheric thermojet (no radioactive release) is probably the cleanest way to go. If given a lifting body for low atm, a hypersonic for the mid-regime, and a release-capsule for trans atmospheric flight, one can attain escape velocity using the ramjet version of thermojet on stage two to hit OV+, and hit the upwards to get there.

Take a lot longer than CT....

Basically, think WhiteKnight/SSOne... but with sealed fusion units for atmospheric work. SSOne is off thrust for almost all of its suborbital flight. Give it a thermo-scramjet, and enough water for minimal thrust maintenance exoatmo, and an intermediate stage which takes it on a thermoram trip to mach 5+ before kickup...

Ports need to be relatively close for economic reasons, and relatively distant for safety reasons, given the traffic flows.


Scott: Remember, in-atmo, one doesn't need more than 0.5G and a clean frame to make multi-mach; it helps, but isn't essential. Given a high efficiency heat source, a thermo-ramjet is free of delta-V limits in atmo. AS the density decreases, top speed increases... as top speed increases, lift is maintained to a lesser degree, but one can get to suborbital velocities in the upper atmosphere on compressed medium alone, if the exhaust and intakes are correct.
 
Bromgrev

The issue is the difference between force (mv) and energy (mv^2)

if your ship is half fuel, then the average exhaust velocity of your fuel is equal to your final velocity. (assuming no drag etc.) The average energy of your exhaust is also equal to your ship's final energy

If you want to have more than 1/2 of your ship be fuel, then your exhaust velocity has to go up: if your ship is 1/4 fuel, then the average exhaust velocity needs to be 3X the final velocity of your ship (the ship is now 3x the weight of your propellant) but the average exhaust *energy* is now 9x the final energy of your ship.

As a result, the fuel efficiency of any "reaction" drive is going to be seriously affected by the energy of the exhaust, and the amount of energy required to move that exhaust will always be *more* than the energy of the exhaust, so fuel efficient drives will *always* be energy pigs (chemical rockets "store" that energy in relatively unstable compounds, nuclear rockets "store" it in the matter of the nuclear elements, with einsteins famous e=mc^2)

Once you realize that orbital velocity is in kilometers per second these numbers get big, fast, and current launch vehicles are lucky if they get 5% of their mass into orbit. HEPLAR exhaust is basically "energetic" enough that in the neigbourhood of 1% of the matter put in must be converted to energy to drive them, which is significantly better energy conversion that the most efficient nuclear devices currently available.

I hope that this physics refresher is what you were asking for, if not, apologies for "lecture mode"

<EDIT>
Note that the chief difference between "Nuclear bomb" and "Nuclear Rocket" is the sameas that between a block of C4 and a tank of gas in a car: the bomb releases all of its energy in a fraction of a second, while the rocket sustains its energy release for much longer periods (hours for the car, days or weeks for the rocket if we figure out how to do it right...)

Anyone hit by the car (or rocket) is just as dead as if they were standing next to the bomb.
</EDIT>

Scott Martin
 
Ah, the cobwebs begin to clear ... Thanks for the reminder! Should have remembered the correlation between rockets and guns.
 
Originally posted by Scott Martin:
Ptah:

I am suitably chastized (and feeling dumb) for not remembering vapour pressure inside the accellerator chamber.

If you have <1G of thrust you don't get off the ground. even if you are using a lifting body you're going to lose the lifting characteristics of the atmosphere long before you get a significant gravity reduction due to altitude.

The radiation belts are not an issue if you can't get there ;)
Click to expand...
Most assuredly. Getting the 1G to lift off with an ion engine is the big problem, and even if you could the amount of fuel consumed might make them impractical compared to chemical means.

Speaking of chemical means, I was going to cite
http://www.projectrho.com/rocket/rocket3c2.html
but I notice that the data listed for real world spacecraft (Space Shuttle, Saturn V) exceeds the Chemical MAX listing which would be forbidden under Mr. Chung's recommended use of his table. ;)

Absent some environmentally friendly nuclear-like drive, getting to orbit at low TL with a "hard" approach will IMHO be reliant on chemical rockets at least for people. Although the % payload might be low compared to CT, advances in fuels and manufacturing might really bring the cost down compared to present day costs.

Now for cargo, at least robust cargo, one might revive the idea of using a canon, but instead of a chemical propellant it is electromagnetic. Absurdly inefficient but the power plant resides on the ground. I don't know how big it would have to be but the image of a 1000 meter tower situated at the equater shooting cargo cannisters into obit makes me smile. Sort of a short bean-stalk. ;)


Ion drives tend to accellerate heavy nucleii at similar velocities, which (IMO) would become an isotope enhancing particle accellerator putting you back to "environmentally unfriendly" ;)

Scott Martin
Click to expand...
Well not that high of velocity, at least not yet for Xe drives.
Otherwise I'd be
toast.gif
(been waiting to use that graemlin).
 
Originally posted by Scott Martin:
Bromgrev

The issue is the difference between force (mv) and energy (mv^2)
Click to expand...
Agreed. You will also see these decribed as thurst and power as rocketry often replaces m with dm/dt, the amount of propellant mass ejected per unit time. Acceleration is determined by dm/dt and power (energy) is typically dominated by v. So you will see a lot of dicussion of the trade off betwwen thrust and v, which can also be phrased as a trade off between acceleration and v. The current spacwe program is pretty fixed on how they get things into orbit drive wise (they are still working on improving the economics of the vehicle). Efficient drive improvements are focused on the orbit-to-orbit part of the mission. NASA wants this drive to be pound-per-pound the most efficient because the cost of getting that pound into orbit is high. They really don't care about acceleration since space-combat is not on their mind. Anyway, don't we have an agreement with the greys on proctecting the planet after we returned their crashed spacecraft in 1948? ;)
(That last comment is purely in jest, but I'm sure it exists seriuosly somewhere on the web.)
 
Ptah:

NASA has been fixated on NOT progressing on in-atmosphere drives due, in part, to the split between the CAA (Now FAA) and NASA.

One potential is getting some Ion drives on SS1's descendants... Use props/jets to get up high, rockets to get up to speed & exosphere, and ion to put into LEO.

And, better, SS1 proved that reentry need not be ablative. An idea which NASA had been saying was nigh on impossible without gravitics.
 
Originally posted by Aramis:
Ptah:

NASA has been fixated on NOT progressing on in-atmosphere drives due, in part, to the split between the CAA (Now FAA) and NASA.

One potential is getting some Ion drives on SS1's descendants... Use props/jets to get up high, rockets to get up to speed & exosphere, and ion to put into LEO.

And, better, SS1 proved that reentry need not be ablative. An idea which NASA had been saying was nigh on impossible without gravitics.
Click to expand...
Yep, what I recall, the space plane still gets floated. When I got out of the field unmanned reusable rockets were the big push, people (FAA?) were just a little leary of having these things come down unmanned. The plasma induced communications blackout on reentry was the scary part. Ion drives were for interplanetary travel.

You know that nea saying of non-ablative rentry was just to get DOD to release the alien gravatic technology from Area-51, don't you? ;)
 
For whatever reasons, The United States Government seems dead set upon keeping us all grounded.

I wonder if we can blame Robert Heinlein?
 
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